The present invention relates to silicone compositions which are substantially free of volatile organic compounds (VOC) and useful as pressure sensitive adhesives (PSAs). More particularly, the present invention relates to the use of silicone fluids in the form of linear, or cyclic organosiloxanes, as non-polluting recyclable carriers in silicone PSA compositions.
As shown by Wengrovious et al., U.S. Pat. Nos. 5,357,007, and 5,324,806, solventless PSAs can be made by blending a spray dried benzene soluble copolymer of SiO.sub.2 units and (R).sub.3 SiO.sub.1/2 units with a fluid network mixture of an alkenyl siloxane and silicon hydride and a hydrosilylation catalyst, where R is a C.sub.(1-13) monovalent organic radical. Volatile organic compounds, such as toluene, can be eliminated by the spray drying of an organic solvent dispersion of an organosiloxane hydrolyzate of SiO.sub.2 units, or "Q", chemically combined with (R).sub.3 SiO.sub.1/2 units, or "M", to produce a silicone compatible MQ resin powder which has an organo to silicon ratio of about 0.5 to about 1.5.
Additional techniques are constantly being evaluated to provide commercial mixtures useful in making silicone pressure sensitive adhesives which are free of polluting, and volatile organic compounds.
The present invention is based on the discovery that a recyclable volatile silicone fluid having a boiling point of 250.degree. C., or below, such as a linear organosiloxane, or a cyclic organosiloxane, can be used effectively as a solvent for a silicone PSA composition. For example, the organic solvent dispersion of an organosiloxane hydrolzate of SiO.sub.2 units and (R).sub.3 SiO.sub.1/2 units made by the method of Daudt et al., U.S. Pat. No. 2,676,182, can be vacuum-dried, and the resulting MQ resin can be redissolved in a volatile silicone fluid as previously discussed. A high viscosity organopolysiloxane, for example, a silicone gum having a viscosity of at least 500,000 centipoise, then can be combined with the silicone fluid-MQ resin solution and further processed to form an environmentally safe silicone PSA composition. In addition, the silicone PSA compositions of the present invention, are preferably free of SiH containing organosiloxane which is required in PSA compositions based on a hydrosilylation cure.